besides 'test.jdk' property, jtreg also sets 'compile.jdk' which points to 'the JDK being used to compile the tests. By default, this is the same as test.jdk and TESTJAVA'. in some cases it's important for tests to use 'compile.jdk' if one is provided, e.g. to get the JDK which has javac. therefore it makes sense to have a separate constant in common testlibrary.